The invention relates to a method of imaging a mask pattern, present in a mask, on a substrate by means of EUV radiation, using a mirror projection system and to amethod for manufacturing devices.
The invention also relates to a lithographic apparatus and to a mask which are suitable for performing the method.
A lithographic apparatus is used, inter alia, in the manufacture of integrated electronic circuits, or ICs, for imaging an IC mask pattern, present in a mask, each time on a different IC area of a substrate. This substrate, which is coated with a radiation-sensitive layer, provides space for a large number of IC areas. The lithographic apparatus may also be used in the manufacture of other devices, for example, liquid crystalline display panels, integrated or planar optical systems, charge-coupled detectors (CCDs) or magnetic heads.
Since it is desirable to accommodate an increasing number of electronic components in an IC, increasingly smaller details, or line widths, of IC patterns must be imaged. Thus, increasingly stricter requirements are imposed on the imaging quality and the resolving power of the projection system which is usually a lens system in the current lithographic apparatuses. The resolution, which is a measure of the smallest detail which can still be imaged satisfactorily, is proportional to .lambda./NA, in which .lambda. is the wavelength of the imaging, or projection, beam and NA is the numerical aperture of the projection system. To increase the resolution, the numerical aperture may, in principle, be increased and/or the wavelength may be reduced. In practice, an increase of the numerical aperture, which is currently already fairly large, is not very well possible because this reduces the depth of focus of the projection lens system, which is proportional to .lambda./NA.sup.2, while it becomes too difficult to correct for the required image field.
The requirements to be imposed on the projection lens system may be alleviated, or the resolution may be increased, while maintaining these requirements, if a step-and-scanning lithographic apparatus is used instead of a stepping lithographic apparatus. In a stepping apparatus, a full-field illumination is used, i.e. the entire mask pattern is illuminated in one operation and imaged as a whole on an IC area of the substrate. After a first IC area has been illuminated, a step is made to a subsequent IC area, i.e. the substrate holder is moved in such a way that the next IC area is positioned under the mask pattern, whereafter this area is illuminated, and so forth until all IC areas of the substrate are provided with the mask pattern. In a step-and-scanning apparatus, only a rectangular or circular segment-shaped area of the mask pattern and hence also a corresponding sub-area of a substrate IC area is each time illuminated, and the mask pattern and the substrate are synchronously moved through the illumination beam, while taking the magnification of the projection lens system into account. A subsequent area of the mask pattern is then each time imaged on a corresponding sub-area of the relevant IC area of the substrate. After the entire mask pattern has been imaged on an IC area in this way, the substrate holder performs a stepping movement, i.e. the beginning of the next IC area is moved into the projection beam and the mask is set to its initial position whereafter said next IC area is scan-illuminated via the mask pattern.
If even smaller details are to be imaged satisfactorily with a step-and-scanning lithographic apparatus, it is still possible to reduce the wavelength of the projection beam. In the current step-and-scanning apparatuses, deep UV (DUV) radiation, i.e. radiation having a wavelength of the order of several hundred nanometers, for example 245 nm or 193 nm from, for example, an excimer laser is already used. Another possibility is the use of extreme UV (EUV) radiation, also referred to as soft X-ray radiation, with a wavelength in the range of several nm to several tens of nm. Extremely small details, of the order of 0.1 .mu.m or less, can be satisfactorily imaged with such a radiation.
Since there is no suitable material with which lenses can be made available for EUV radiation, a mirror projection system must be used for imaging the mask pattern on the substrate, instead of a hitherto conventional projection lens system. For forming a suitable illumination beam from the radiation of the radiation source unit, mirrors are also used in the illumination system.
The article "Front-end design issues in soft X-ray projection lithography" in Applied Optics, vol. 32, no. 34, Jan. 12, 1993, pp. 7050-56 describes a lithographic projection apparatus in which EUV radiation is used. The illumination system of this apparatus comprises a condenser system in the form of three mirrors and the imaging, or projection, system comprises four mirrors. The radiation source unit comprises a high-power laser generating a plasma in a medium emitting EUV radiation. This radiation source unit is known as Laser Produced Plasma Source (LPPS). Said medium may be a solid, a liquid or a gaseous medium, and the generated EUV radiation has a wavelength of 13 nm.
It is a great problem in EUV lithographic apparatuses to illuminate the substrate with a sufficiently high intensity. A first cause of this problem is that radiation sources emitting radiation at the envisaged wavelength, in the range of 13 nm, are not very efficient and only supply a limited quantity of radiation. Moreover, the mirrors are considerably less than 100% reflecting. Each of these mirrors has a multilayer structure whose composition is adapted as satisfactorily as possible to the wavelength of the projection beam used. Examples of such multilayer structures are described in U.S. Pat. No. 5,153,898. A multilayer structure which is often referred to in literature is the structure consisting of silicon layers alternating with molybdenum layers. For radiation supplied by a plasma source, these layers theoretically have a reflection of the order of 73% to 75%, but in practice, the reflection is currently in the order of 65%. When said number of seven mirrors is used with a reflection of 68% each, only 6.7% of the radiation emitted by the source reach the substrate. In practice, this means for a lithographic apparatus that the illumination time must be relatively long so as to obtain the desired quantity of radiation energy on an IC area of the substrate, while for a scanning apparatus particularly the scanning rate is relatively short. However, it is essential for these apparatuses that the scanning rate is as high as possible and the illumination time is as short as possible so that the throughput, i.e. the number of substrates which can be illuminated per unit of time, is as high as possible.